The present invention pertains to a radar enhancing transponder and particularly to a transponder for use with a radar in which the transmitted signal is amplitude modulated and the percent of modulation is the desired or useful information which must be retransmitted to the radar. In particular, such radars are included in landing systems wherein the radar incorporates conical scan angle tracking transponder located on the airplane attempting to land receives a signal on which the amount of modulation is dependent upon the distance the airplane is from the center of the conical scan. In this system the percent of modulation, rather than a specific amount of amplitudemodulation, is the critical characteristic which provides the desired information. Thus, it is important that the reply signal transmitted by the transponder on the airplane has the correct percentage of modulation.
Insuring that the reply signal transmitted by the transponder has the correct percentage of modulation thereon is further complicated by the fact that the reply signal is generally transmitted at a different frequency than the signal received from the radar. For example, the radar signal may lie in the Ka band of frequencies while the reply signal may lie in the X band.
In the present transponders the modulation of the reply signal is accomplished by striping the modulation signal off of the received signal, and using it to drive a modulator in the transponder. This is an open loop approach to reproducing the received modulation signal on the reply signal. An automatic gain control amplifier is used in the receiver to maintain a near constant signal amplitude at the input to a modulation detector. The circuit which recovers the modulation from the received signal does not measure percent modulation. If the AGC amplifier in the receiver were perfect, it could be argued that the recovered modulation signal is a measure of percent modulation, but this is obviously not the case. Further, the modulator which modulates the reply signal does not always convert its drive signal into a correct modulation of the X band signal. The device used to modulate the X band signal is a pin diode attenuator. Pin diodes are nonlinear, and the attenuation versus drive signal amplitude varies widely from unit to unit. Further, the X band power produced by the transponder can vary by at least a factor of two from unit to unit. Since there is no way to compensate for this variation between units, it can potentially produce a significant difference between the received and output percentage modulation.